Superimposed carrier telephone and carrier telegraph system



H, A, AFI-EL 1,932,568

SUPERIMPOSD CARRIER TELEPHONE AND CARRIER TELEGBAPH SYSTEM Oct. 31,1933'.

Filed June 20, 19251 .NNN

INVENTOR magy/ff@ ATTORNEY Patented Oct. 31, 1933 SUPERIMPOSED CARRIERTELEPHONE AND CARRIER TELEGRAPH SYSTEM Herman A. Affel, Ridgewood, N.J., assignor to American Telephone a corporation of New and TelegraphCompany,

York

Application June 20, 1931. SerialNo. l545,749

2 Claims.

This invention relates to superimposed carrier telephone and carriertelegraph systems, and particularly to a method and means for preventingthe carrier telephone currents from causing interference to the carriertelegraph signals when,- ever the said telephone currents exceed apredetermined value.

In a carrier signaling system in which a plurality of channels areoperated over to the same transmission line, it is customary to amplifythe currents of all channels by passing them through the same amplier ateach repeater station along the line. No diiculty is experienced withthat mode of operation as long as all channels are employed for the samepurpose, that is, if all transmit telephone signals, or all telegraphsignals.

It is sometimes desirable, however, to employ one or more of thechannels of a multi-channel carrier telephone system for telegraphpurposes, that is, to transmit over the selected channel a plurality oftelegraph modulated waves. Experiments made upon such a combined systemhave shown that when the combined currents of the telephone and thetelegraphy waves were amplied by the same repeated the telegraphoperation suffered and frequent false signalsl were pro-` duced. Thatdifliculty arose from the fact that whenever the -telephonic currentsexceeded a predetermined limit, the amplifiers at the repeater stationswould be overloaded and the gain of the amplifier would drop. That wouldcause a'corresponding drop in the amplitude of the telegraph currentsand wouldproduce the false operation of the relays at the terminals.That situation could be remedied either by lowering the level of thetelephone currents passing through the amplifier very substantially,that is, 15 to 20 db., or an ampliiier could be provided having muchlarger tubes than would be necessary for normal operation. The rstalternative is undesirable from the standpoint of plant operation,because of the 'resulting greater interference from external sources,and theflatter is uneconomical because of the greater cost of equipmentand maintenance.

The object of this invention is to provide a system whereby theoccasional excessive telephonic currents or voltages may be eliminatedbefore they reach the common amplifiers at the repeater stations, thuspreventing the detrimental eiect upon the telegraph channelshereinbefore described.

This invention will be apparent from the following description when readin connection with the `attached drawing showing schematically a simpleform of embodiment of the invention.

In the drawing, A, B and C represent three terminal circuits by whichsignal modulated Waves are produced and impressed upon the line L fortransmission to a distant station, which distant station would.preferably have terminal equipment similar to that shown in the drawing.Connected between sections of the line L is a re'- peater R of whichonly `the high frequency branch is shown in detail,` since it is withthat portion that the present invention is concerned. The terminalcircuits A and B are arrangedv to transmit and receive telephonesignals, and the terminal circuit C is adapted for sending and receivingtelegraph signals. Circuit A comprises a telephone set 1 which isconnected with the line windings of the hybrid coil 2.y The trans--vmitting branch which is bridged across the line windings, includes amodulator 3, a current limit.- ing device 4, and a band lter 5 intendedto transmit the range of frequencies constituting onev of the telephonechannels. The device 4 includes a gas filled, or vacuum discharge deviceoperable at a predetermined voltage, which is applied by the transformervwhose primary *winding` is bridged across the transmitting path ofcircuit A. The band lter 5 is connected With the com mon transmittingbranch 6 having therein an amplifier 7 and a iilter 8. The outputof thatlter is connected to the line L by the branch circuit 9 which includes ahigh pass filter 10. The receiving branch of the terminal circuit Aincludes a band filter 11, and the demodulator 12, the said band filterbeing designed to pass the range of frequencies intended for receptionby the circuit A. The receiving branch of circuit A is connected Withthe common receiving branch 13 which includes the filter 14 and thereceiving amplifier 15. lIt will be seen that the modulated wavesoutgoing from circuit A pass -to vthe branch 6 where they are ampliedand impressed upon the branch 9` which in turn impresses those Wavesupon the line L. The incoming waves from the line L pass to the branchcircuit 9 thence to the branch circuit 13, and those waves of thefrequency transmittible through the filter 11 will be demodulated by 12and will pass to the telephone receiver. i 1

The terminal circuit B, in which the parts similar to those of circuit Ahave been designated by the same numerals primed, has its transmittingbranch connected with the common transmitting branch 6, and itsreceiving branch connected with the common receiving branch 13.

IHC

Since circuit B functions in the same way as circuit A, furtherdescription is not necessary.

The terminal circuit C also comprises sending and receiving branches.The sending branch, namely the upper one shown in the drawing, comprisescarrier sending apparatus 16- (which may be of any well known type) towhich a plurality of telegraph sending circuits are connected. Theoutput of the carrier sending apparatus is connected to the input of themodulator 17, the connection including the sending iilters 18. Theoutput of the modulator 17 is connected to the branch circuit 6, theconnection including the band filter 19, The receiving branch of circuitC which is bridgedacross the receiving branch 13, includes the band lterand the demodulator 21. The outputof the demodulator is connected withthe carrier receiving apparatus 22, the connection including thereceiving filters 23. The output of the carrier receivingv apparatus isconnected with the telegraph receiving-` circuits. f The repeater R isshown as a two-way two element repeater, the upper branch comprisinglters and an amplier for transmitting in one direction, and the lowerbranch being similarly equipped'for transmitting in the oppositedirection.

' The object of the invention, and the manner in which that object isattained, will be apparent from the following description of theoperation of the circuits hereinbefore described. The telephoniccurrents produced by the transmitter of 'sub-station 1 of circuit A willbe impressed upon the modulater 3, and the modulated wave willbetransmitted through the band lter 5 to the transmittingbranch 6 whereinit will be ampli- 'ied by 7 and impressed upon the line L after passingover the branch 9 and through the high pass-"filter 10. If thetransmitter of sub-station 1 is operated at the same time, the waveproduced by the modulator 3 will also pass to the common branch circuit6 and when ampliiied by 7 will be impressed upon the line L. Likewise,the

telegraph signals which are impressed upon the modulator 17 will producea plurality' of waves that will be impressed upon the branch 6, and whenamplified by 7 will also be impressed upon the line L. All ofthosefcurrents will pass through the highfpass filter 24 at the repeaterR, and will in' turn pass through the amplier in the upper branch` ofthat repeater. If, during the trans- 'mission' of those signals, thetelephonie vibrations produced by one of the sub-stations exceeds apredetermined limit, the modulated current resulting therefrom willlikewise exceed its limit,

'and as a result thereof the amplifier 25v may be gas filled tube,having characteristics such that ionization takes place at potentialsbeyond a certain applied voltage, and the impedance of the device,otherwise fairly high, is for those overloaded conditions, lowered to agure approaching zero. Inasmuch as the potential required to initiatethe operation of` those devices is ordinarily fairly high, that is ofthe order of 100 to 200 volts, the device, as illustrated, requiresastep-up transformer in order that it will function. atthe lowervoltages normally employed upon modulators used in telephonetransmission circuits. It may be noted that the arrangement will alsofunction if the current limiting device is connected in the circuit atthe input to the modulator. f

It is to be understood however, that the invention is not limited to theparticular type of current limiting device shown in the drawing anddescribed herein, inasmuch asV otherdevices may beV employed toaccomplish'the object of this invention, namely, to'effectively cut offthose peaks of the wave corresponding to vibrations of the telephonetransmitter having excessive amplitude.

It will accordingly be seen, with the use of devices for limiting theamplitude of the telephonically modulated waves, the amplifier 25 at therepeater R will not be overloaded, and the detrimental effect heretoforeexperienced upon the telegraph channel superimposed upon the telephonechannels, is substantially' eliminated.

While this invention has been -disclosed as embodied in a particularform, it is to be understood that such showing is simply to illustratethe invention, which is capable of embodiment in other anddilerent'iorms without departing from the spirit and scope of theappended claims.

What is claimed is:

1. In a multi-channel carrier telephone system having a carriertelegraph system superimposed thereon, the combination with atransmission line having therein a repeater, of a terminal circuithaving means to modulate a carrier wave by telephone vibrations and toimpress the modulated wave upon thetransmission line, another terminalcircuit having means -to modulate another carrier wave by telegraphsignals and to impressV such modulated wave upon said transmission line,the said repeater of the transmission line being arranged to amplifyboth modulated waves, and means connected with the output circuit ofthe'modulating means of the lfirst mentioned terminal circuit to limitthe amplitude of the telephonically modulated waves prior to theirimpression upon the transmission line.

2. In asystem in which carrier telephone and carrier telegraph signalsare transmitted over the Same path and ampliiied by the same repeater,the combination with a transmission circuit of a repeater connectedtherewith, means 'to apply carrier telegraph signals to saidtransmission circuit, means to apply carrier telephone signals to saidcircuit, the latter means including a modulator and means connected theoutput` cirico izo

@uit of the said modulator to limit the amplitude of the modulated waveto a predetermined level.

HERMAN A. AFFEL.

